There are many circumstances under which it is desirable to be able to measure or monitor the attitude or position of a first body with respect to a second body. When material connections can be made between the two bodies, the changes in attitude or position are quite easily detected. It sometimes happens, however, that there is no convenient or practical way of providing material connections between the bodies, so that alternative means for detecting the changes of interest must be sought. Observation of the movement of an aircraft wing under various conditions of loading, wind gusting, and vehicle maneuvering is one example of such a situation, as will now be explained.
The intuitive concept of an aircraft as a rigid structure moving through the air is fallacious, especially in connection with more modern aeronautical designs. Very considerable motion is possible, for example, of the wing tips with respect to the fuselage: they are capable of pitch motion, when the wing twists about a generally transverse axis, of roll motion, when the wing bends upward or downward, and even of some yaw motion. The latter motion is more particularly significant for a wing store, that is, for an object mounted on the upper or lower wing surface such as a missile.
It is desirable to be able to monitor, or compensate for, the wing geometry changes which occur--in flight testing, in wind tunnel experiments, and in tactical situations involving discharge of wing-born missiles. By the way of illustration certain problems related to military aircraft will be discussed below.
Survival of interceptor and ground attack aircraft is very difficult when the enemy possesses modern defensive weapons. A possible way to deal with the threat is to fly the aircraft on an unpredictable course using high-g maneuvers. Most recent analyses indicate that four seconds is about the maximum nonmaneuvering time that can be tolerated, and that maneuvers involving two to four g's should be used during or within 4 seconds of weapon firing. This implies that remote parts of the fire control system will be stressed out of alignment during critical phases of target acquisition and pointing.
Reconnaissance systems often involve an attitude tie-in to an attitude reference in the fuselage, and yet they may be mounted, so as to be quite movable with respect thereto, through flexibility of the wings.
Gusts and maneuvers frequently cause disturbances in the wings that reduce the margin of safety enough to limit maneuvers. Wing bend and twist often misalign a missile having a slewable tracking seeker. Such a missile usually has a narrow field of view, often only two or three degrees wide, while the wing bend and twist can be as much as nine degrees each, so that a target picked up on a fuselage-mounted radar or electrooptical system cannot be located within the field of view of the missile itself.
The problem also exists when seeker pods are mounted under the wing because they may be stressed out of alignment with the fuselage-mounted fire control elements and other wing mounted elements. They are also large and heavy, and thus tend to induce aerodynamic loads that cause both steady state and transient misalignments.